Schmitt triggers are often used to increase noise immunity for noisy and/or slow moving signals. This is achieved by providing hysteresis which effectively raises the threshold voltage (or switch point) when a logic low is present and lowers the switch point when a logic high is present. A conventional CMOS Schmitt trigger uses a weak P channel device and a weak N channel device for feedback to change the switch point of the circuit. The amount of hysteresis is adjusted by adjusting the gain of the feedback transistors.
It has been found, however, that at low power supply voltages the feedback transistors must be very weak in order for the circuit to switch at all, or at least within acceptable limits. If very weak devices are used, there is very little hysteresis at higher voltages. Consequently, if it is desirable to be able to have an operating range of several volts in which a relatively low voltage is the low end of the range, the amount of hysteresis can only be very small.